Power tongs have been around for many years, and efforts to control the output torque are not new. The major problem with torque control has been related to the inertia involved in the system including the pipe being rotated. Threaded joints occur about every thirty feet in a pipe string, and several hundred are involved in a common well bore related installation. Economic considerations urge speed in pipe joint make-up, and this speed contributes to the problems involved. Pipe threads are commonly tapered, and most have shoulders. Jamming the shoulders together with the pipe rapidly spinning is called "bumping," and such bumping is destructive.
In recognition of the bumping problem, many organizations have demanded recordings made of torque applied to pipe joints. There is no proven way, however, to record from torque load cells, the torque supplied from the energy of a rapidly spinning pipe suddenly shouldered to the mating thread.
To compensate for the lack of control over torque applied to pipe used, for instance, in hydrogen sulfide service, the pipe is often rotated by hand until the shoulder is approached. Powered tongs usually then complete the torque program.
Efforts to control torque have involved sensors to trigger drive motor shut down when torque reached reselected values. Since pipe joints often wobble about as assembly proceeds, the triggering torque can be sensed from jamming before the shoulders are in contact. Hopefully, such conditions are always detected, and additional torque efforts can correct the impending problem. This, however, is risky and costly.
It is desireable to have a power tong that will slow down as torque is sensed, even to the point of stopping when desired torque is achieved, yet hold the torque through brief jamming incidents and finish the task, even if motion is intermittent.
It is therefore an object of this invention to provide apparatus to reduce the speed of power tongs as soon as torque resistance is sensed.
It is a further object of this invention to provide apparatus to limit torque that is delivered by power tongs, yet hold the torque applied as long as the power tong operator holds the power tong manual control valve open.
It is still another object of this invention to provide apparatus to control power tong torque that will stop power tong rotation when unstable conditions cause transient resistance, yet continue rotation when such transients pass until the connection is satisfactorily completed.
It is yet a further object of this invention to utilize a conventional hydraulic load cell in the tong tension line to directly and proportionately regulate the means to limit the ability of the power tong drive motor to deliver both space and torque.
It is still another object of this invention to provide apparatus to slow the speed of a power tong as torque is first sensed, and to finally limit torque as rotation of pipe gradually ceases.
These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached drawings and appended claims.